How do i report / tip off a security clearance holder involved in fraud?

If you know what agency the individual works for then you should contact the security office there and ask to speak with the highest ranking person in charge. Ensure you have the pertinent facts, not 3rd party information or gossip, but information that you know first hand. Who, what, when, where and how, and if you know the why. The more facts you can bring to the table the more seriously you will be taken. Ensure you document who you talked to and when, in case it comes up in some other context. If the person is a contractor then contact the government agency that is responsible for the work they are doing.